Smaller, Faster, Cheaper Synthesizers
One of the trends in today’s electronic instruments evolution is that technology is getting smaller, faster and cheaper every day. This trend spreads from a smartphone (that is smaller but more powerful than a typical PC of just a decade ago) to complex RF/microwave measurement instruments. The frequency synthesizer is a key element that generates a stimulus signal or is used as a local oscillator in a variety of up- and down-conversion schemes. The industry feels persistent pressure to deliver higher-performance, higher-functionality, smaller-size and lower-cost synthesizer designs. Obviously, wide frequency coverage and small step size are the key synthesizer design targets. Aside from frequency coverage and resolution, a synthesizer’s spectral purity (i.e., phase noise and spurs) is the primary evil that ultimately limits the performance of any system. More recently, switching speed has become a significant player in this game as well. Newer RF/microwave systems require faster switching due to the ongoing increase of data flow. Finally, everything should be packed into a tiny footprint that can support new instrument platforms or design ideas.
These design targets present certain technological challenges. Historically, high-performance PLL synthesizers have relied on YIG-tuned oscillators featuring broadband operation and excellent phase noise characteristics. However, the high power consumption, large size and especially slow tuning speed, inherent to YIG oscillators, have contributed to a shift to solid-state VCO architectures. VCO-based synthesizers are significantly faster; however, their phase noise has traditionally been considered to be inferior when compared to YIG-based designs.
To address these requirements, Phase Matrix introduced the QuickSyn series of microwave frequency synthesizers in late 2008. The employed patented architecture provides a unique combination of fast-switching speed and low phase noise characteristics. The main idea is to substitute a slow-tuning, bulky and expensive YIG oscillator with a tiny VCO that can easily support microsecond tuning. Excessive phase noise (traditionally associated with VCO devices) is washed out by utilizing an ultra wideband PLL scheme in conjunction with a low-noise reference source. Thus, the QuickSyn synthesizer combines microsecond-range tuning and low phase noise, which is somewhat comparable to top-rated signal generators. Although the design is essentially a fraction of the size of a traditional bench-top or rack-mount instrument, it offers many of the same features as larger units. In fact, the QuickSyn provides all major modulation capabilities (AM, FM, phase and pulse), power leveling and control, frequency and power sweep, list mode, and many other functions that are usually found in complex test and measurement instruments.
Despite this high performance, the first QuickSyn synthesizer still leaves room for further improvements. The new, recently introduced QuickSyn Lite synthesizers are less than half the size of the full featured models, yet offer remarkable characteristics. This product is available in two models, the FSL-0010 and FSL-0020, covering the 0.65 to 10 GHz and 0.65 to 20 GHz ranges, respectively. Both models utilize broadband solid-state VCOs that offer fundamental output to 10 and 20 GHz, respectively. In contrast to widely used frequency multiplication schemes, this approach eliminates possible spectrum contamination by subharmonic products. The VCO coverage is extended down by utilizing a frequency divider that improves phase noise and spurious characteristics at lower frequencies as shown in Figure 1. The use of the advanced DDS approach (in conjunction with dedicated spur-reduction circuitry) enables a very fine frequency resolution of 0.001 Hz without a common penalty of slower tuning speed or elevated spurs. The utilized PLL hardware itself needs just a few tens of microseconds to bring the output frequency to a desired value while the output is completely locked and refined within less than a hundred microseconds. Digital signal processing adds extra delays required to receive a tuning command, perform all necessary calculations in accordance with the employed frequency plan, and program individual devices. Hence, the total switching time is specified at 200 µsec in the regular operation mode when new frequency commands are sent one by one. Most of these delays, however, can be reduced or completely eliminated in the list mode. The switching speed in the list mode is specified at 100 µsec regardless of the current and destination frequency (i.e., the specification is valid from “any to any” frequency step within the entire operating range).
The synthesizer includes an internal dual-oscillator reference that is composed by combining TCXO and VCXO devices. This results in excellent thermal stability and low phase noise without a common penalty of large size and high power consumption of OCXO devices. The internal reference is factory calibrated to a GPS standard to ensure adequate accuracy of the synthesized signal. The synthesizer supplies a 10 MHz reference signal to the outside world. The internal oscillator can be automatically locked to an external reference too. The synthesizer also provides the ability to adjust the internal oscillator frequency (via software) for temperature and aging compensation as desired.
Both models include SPI and USB control interfaces and are immediately deployable by connecting them to a personal computer and power source. A soft front panel allows the user to access all synthesizer functions. The synthesizer is shielded in a small metal box measuring 4" × 4" × 0.8". It is biased from a single +12 V DC supply. The built-in self test monitors the synthesizer’s internal temperature and voltages as required. Overall, the exceptional performance and extended functionality make the QuickSyn Lite synthesizer an ideal building block for a variety of instruments and subsystems. Designers restricted by reduced footprint goals, power consumption constraints and the need for low phase noise will appreciate the QuickSyn Lite synthesizer as the single solution to a multitude of design challenges.
a National Instruments Company,
Santa Clara, CA